/*
 * Copyright (C) 2019 Qunar, Inc.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

package qunar.tc.bistoury.serverside.agile;

import com.google.common.base.Charsets;

import java.io.UnsupportedEncodingException;

//This code was converted from code at http://iharder.sourceforge.net/base64/
//Lots of extraneous features were removed.
/* The original code said:
 * <p>
 * I am placing this code in the Public Domain. Do with it as you will.
 * This software comes with no guarantees or warranties but with
 * plenty of well-wishing instead!
 * Please visit
 * <a href="http://iharder.net/xmlizable">http://iharder.net/xmlizable</a>
 * periodically to check for updates or to contribute improvements.
 * </p>
 *
 * @author Robert Harder
 * @author rharder@usa.net
 * @version 1.3
 */

/**
 * Base64 converter class. This code is not a full-blown MIME encoder;
 * it simply converts binary data to base64 data and back.
 */
public final class Base64 {

    /** The equals sign (=) as a byte. */
    private final static byte EQUALS_SIGN = (byte) '=';

    /** The new line character (\n) as a byte. */
    private final static byte NEW_LINE = (byte) '\n';

    private final static byte[] ALPHABET = { (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F',
            (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K', (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O',
            (byte) 'P', (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U', (byte) 'V', (byte) 'W', (byte) 'X',
            (byte) 'Y', (byte) 'Z', (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e', (byte) 'f', (byte) 'g',
            (byte) 'h', (byte) 'i', (byte) 'j', (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o', (byte) 'p',
            (byte) 'q', (byte) 'r', (byte) 's', (byte) 't', (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y',
            (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3', (byte) '4', (byte) '5', (byte) '6', (byte) '7',
            (byte) '8', (byte) '9', (byte) '+', (byte) '/' };

    private final static byte[] WEBSAFE_ALPHABET = { (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E',
            (byte) 'F', (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K', (byte) 'L', (byte) 'M', (byte) 'N',
            (byte) 'O', (byte) 'P', (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U', (byte) 'V', (byte) 'W',
            (byte) 'X', (byte) 'Y', (byte) 'Z', (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e', (byte) 'f',
            (byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j', (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o',
            (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't', (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x',
            (byte) 'y', (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3', (byte) '4', (byte) '5', (byte) '6',
            (byte) '7', (byte) '8', (byte) '9', (byte) '-', (byte) '_' };

	/**
	 * Translates a Base64 value to either its 6-bit reconstruction value
	 * or a negative number indicating some other meaning.
	 **/
	private final static byte[] DECODABET = {-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
		-5, -5, // Whitespace: Tab and Linefeed
		-9, -9, // Decimal 11 - 12
		-5, // Whitespace: Carriage Return
		-9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
		-9, -9, -9, -9, -9, // Decimal 27 - 31
		-5, // Whitespace: Space
		-9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 42
		62, // Plus sign at decimal 43
		-9, -9, -9, // Decimal 44 - 46
		63, // Slash at decimal 47
		52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
		-9, -9, -9, // Decimal 58 - 60
		-1, // Equals sign at decimal 61
		-9, -9, -9, // Decimal 62 - 64
		0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
		14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
		-9, -9, -9, -9, -9, -9, // Decimal 91 - 96
		26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
		39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
		-9, -9, -9, -9, -9 // Decimal 123 - 127
		/*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
	};

	/** The web safe decodabet */
	private final static byte[] WEBSAFE_DECODABET =
	{-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
		-5, -5, // Whitespace: Tab and Linefeed
		-9, -9, // Decimal 11 - 12
		-5, // Whitespace: Carriage Return
		-9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
		-9, -9, -9, -9, -9, // Decimal 27 - 31
		-5, // Whitespace: Space
		-9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 44
		62, // Dash '-' sign at decimal 45
		-9, -9, // Decimal 46-47
		52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
		-9, -9, -9, // Decimal 58 - 60
		-1, // Equals sign at decimal 61
		-9, -9, -9, // Decimal 62 - 64
		0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
		14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
		-9, -9, -9, -9, // Decimal 91-94
		63, // Underscore '_' at decimal 95
		-9, // Decimal 96
		26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
		39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
		-9, -9, -9, -9, -9 // Decimal 123 - 127
		/*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
     -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
	};

	// Indicates white space in encoding
	private final static byte WHITE_SPACE_ENC = -5;
	// Indicates equals sign in encoding
	private final static byte EQUALS_SIGN_ENC = -1;

	/** Defeats instantiation. */
	private Base64() {
	}

	/* ********  E N C O D I N G   M E T H O D S  ******** */

	/**
	 * Encodes up to three bytes of the array <var>source</var>
	 * and writes the resulting four Base64 bytes to <var>destination</var>.
	 * The source and destination arrays can be manipulated
	 * anywhere along their length by specifying
	 * <var>srcOffset</var> and <var>destOffset</var>.
	 * This method does not check to make sure your arrays
	 * are large enough to accommodate <var>srcOffset</var> + 3 for
	 * the <var>source</var> array or <var>destOffset</var> + 4 for
	 * the <var>destination</var> array.
	 * The actual number of significant bytes in your array is
	 * given by <var>numSigBytes</var>.
	 *
	 * @param source the array to convert
	 * @param srcOffset the index where conversion begins
	 * @param numSigBytes the number of significant bytes in your array
	 * @param destination the array to hold the conversion
	 * @param destOffset the index where output will be put
	 * @param alphabet is the encoding alphabet
	 * @return the <var>destination</var> array
	 * @since 1.3
	 */
	private static byte[] encode3to4(byte[] source, int srcOffset,
			int numSigBytes, byte[] destination, int destOffset, byte[] alphabet) {
		//           1         2         3
		// 01234567890123456789012345678901 Bit position
		// --------000000001111111122222222 Array position from threeBytes
		// --------|    ||    ||    ||    | Six bit groups to index alphabet
		//          >>18  >>12  >> 6  >> 0  Right shift necessary
		//                0x3f  0x3f  0x3f  Additional AND

		// Create buffer with zero-padding if there are only one or two
		// significant bytes passed in the array.
		// We have to shift left 24 in order to flush out the 1's that appear
		// when Java treats a value as negative that is cast from a byte to an int.
		int inBuff =
			(numSigBytes > 0 ? ((source[srcOffset] << 24) >>> 8) : 0)
			| (numSigBytes > 1 ? ((source[srcOffset + 1] << 24) >>> 16) : 0)
			| (numSigBytes > 2 ? ((source[srcOffset + 2] << 24) >>> 24) : 0);

		switch (numSigBytes) {
		case 3:
			destination[destOffset] = alphabet[(inBuff >>> 18)];
			destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
			destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
			destination[destOffset + 3] = alphabet[(inBuff) & 0x3f];
			return destination;
		case 2:
			destination[destOffset] = alphabet[(inBuff >>> 18)];
			destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
			destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
			destination[destOffset + 3] = EQUALS_SIGN;
			return destination;
		case 1:
			destination[destOffset] = alphabet[(inBuff >>> 18)];
			destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
			destination[destOffset + 2] = EQUALS_SIGN;
			destination[destOffset + 3] = EQUALS_SIGN;
			return destination;
		default:
			return destination;
		} // end switch
	} // end encode3to4

	/**
	 * Encodes a byte array into Base64 notation.
	 * Equivalent to calling
	 * {@code encodeBytes(source, 0, source.length)}
	 *
	 * @param source The data to convert
	 * @since 1.4
	 */
	public static String encode(byte[] source) {
		return encode(source, 0, source.length, ALPHABET, true);
	}

	/**
	 * Encodes a byte array into web safe Base64 notation.
	 *
	 * @param source The data to convert
	 * @param doPadding is {@code true} to pad result with '=' chars
	 *        if it does not fall on 3 byte boundaries
	 */
	public static String encodeWebSafe(byte[] source, boolean doPadding) {
		return encode(source, 0, source.length, WEBSAFE_ALPHABET, doPadding);
	}

    /**
     * Encodes a byte array into Base64 notation.
     * 
     * @param source The data to convert
     * @param off Offset in array where conversion should begin
     * @param len Length of data to convert
     * @param alphabet is the encoding alphabet
     * @param doPadding is {@code true} to pad result with '=' chars if it does
     *        not fall on 3 byte boundaries
     * @since 1.4
     */
    public static String encode(byte[] source, int off, int len, byte[] alphabet, boolean doPadding) {
        byte[] outBuff = encode(source, off, len, alphabet, Integer.MAX_VALUE);
        int outLen = outBuff.length;

        // If doPadding is false, set length to truncate '='
        // padding characters
        while (doPadding == false && outLen > 0) {
            if (outBuff[outLen - 1] != '=') {
                break;
            }
            outLen -= 1;
        }

        return new String(outBuff, 0, outLen, Charsets.UTF_8);
    }

	/**
	 * Encodes a byte array into Base64 notation.
	 *
	 * @param source The data to convert
	 * @param off Offset in array where conversion should begin
	 * @param len Length of data to convert
	 * @param alphabet is the encoding alphabet
	 * @param maxLineLength maximum length of one line.
	 * @return the BASE64-encoded byte array
	 */
	public static byte[] encode(byte[] source, int off, int len, byte[] alphabet,
			int maxLineLength) {
		int lenDiv3 = (len + 2) / 3; // ceil(len / 3)
		int len43 = lenDiv3 * 4;
		byte[] outBuff = new byte[len43 // Main 4:3
		                          + (len43 / maxLineLength)]; // New lines

		int d = 0;
		int e = 0;
		int len2 = len - 2;
		int lineLength = 0;
		for (; d < len2; d += 3, e += 4) {

			// The following block of code is the same as
			// encode3to4( source, d + off, 3, outBuff, e, alphabet );
			// but inlined for faster encoding (~20% improvement)
			int inBuff =
				((source[d + off] << 24) >>> 8)
				| ((source[d + 1 + off] << 24) >>> 16)
				| ((source[d + 2 + off] << 24) >>> 24);
			outBuff[e] = alphabet[(inBuff >>> 18)];
			outBuff[e + 1] = alphabet[(inBuff >>> 12) & 0x3f];
			outBuff[e + 2] = alphabet[(inBuff >>> 6) & 0x3f];
			outBuff[e + 3] = alphabet[(inBuff) & 0x3f];

			lineLength += 4;
			if (lineLength == maxLineLength) {
				outBuff[e + 4] = NEW_LINE;
				e++;
				lineLength = 0;
			} // end if: end of line
		} // end for: each piece of array

		if (d < len) {
			encode3to4(source, d + off, len - d, outBuff, e, alphabet);

			lineLength += 4;
			if (lineLength == maxLineLength) {
				// Add a last newline
				outBuff[e + 4] = NEW_LINE;
				e++;
			}
			e += 4;
		}

		assert (e == outBuff.length);
		return outBuff;
	}


	/* ********  D E C O D I N G   M E T H O D S  ******** */


	/**
	 * Decodes four bytes from array <var>source</var>
	 * and writes the resulting bytes (up to three of them)
	 * to <var>destination</var>.
	 * The source and destination arrays can be manipulated
	 * anywhere along their length by specifying
	 * <var>srcOffset</var> and <var>destOffset</var>.
	 * This method does not check to make sure your arrays
	 * are large enough to accommodate <var>srcOffset</var> + 4 for
	 * the <var>source</var> array or <var>destOffset</var> + 3 for
	 * the <var>destination</var> array.
	 * This method returns the actual number of bytes that
	 * were converted from the Base64 encoding.
	 *
	 *
	 * @param source the array to convert
	 * @param srcOffset the index where conversion begins
	 * @param destination the array to hold the conversion
	 * @param destOffset the index where output will be put
	 * @param decodabet the decodabet for decoding Base64 content
	 * @return the number of decoded bytes converted
	 * @since 1.3
	 */
	private static int decode4to3(byte[] source, int srcOffset,
			byte[] destination, int destOffset, byte[] decodabet) {
		// Example: Dk==
		if (source[srcOffset + 2] == EQUALS_SIGN) {
			int outBuff =
				((decodabet[source[srcOffset]] << 24) >>> 6)
				| ((decodabet[source[srcOffset + 1]] << 24) >>> 12);

			destination[destOffset] = (byte) (outBuff >>> 16);
			return 1;
		} else if (source[srcOffset + 3] == EQUALS_SIGN) {
			// Example: DkL=
			int outBuff =
				((decodabet[source[srcOffset]] << 24) >>> 6)
				| ((decodabet[source[srcOffset + 1]] << 24) >>> 12)
				| ((decodabet[source[srcOffset + 2]] << 24) >>> 18);

			destination[destOffset] = (byte) (outBuff >>> 16);
			destination[destOffset + 1] = (byte) (outBuff >>> 8);
			return 2;
		} else {
			// Example: DkLE
			int outBuff =
				((decodabet[source[srcOffset]] << 24) >>> 6)
				| ((decodabet[source[srcOffset + 1]] << 24) >>> 12)
				| ((decodabet[source[srcOffset + 2]] << 24) >>> 18)
				| ((decodabet[source[srcOffset + 3]] << 24) >>> 24);

			destination[destOffset] = (byte) (outBuff >> 16);
			destination[destOffset + 1] = (byte) (outBuff >> 8);
			destination[destOffset + 2] = (byte) (outBuff);
			return 3;
		}
	} // end decodeToBytes

    /**
     * Decodes data from Base64 notation.
     * 
     * @param s the string to decode (decoded in {@code "UTF-8"} encoding)
     * @return the decoded data
     * @since 1.4
     */
    public static byte[] decode(String s) throws UnsupportedEncodingException {
        byte[] bytes = s.getBytes(Charsets.UTF_8);
        return decode(bytes, 0, bytes.length);
    }

    /**
     * Decodes data from web safe Base64 notation. Web safe encoding uses '-'
     * instead of '+', '_' instead of '/'
     * 
     * @param s the string to decode (decoded in {@code "UTF-8"} encoding)
     * @return the decoded data
     */
    public static byte[] decodeWebSafe(String s) throws UnsupportedEncodingException {
        byte[] bytes = s.getBytes(Charsets.UTF_8);
        return decodeWebSafe(bytes, 0, bytes.length);
    }

    /**
     * Decodes Base64 content in byte array format and returns the decoded byte
     * array.
     * 
     * @param source The Base64 encoded data
     * @return decoded data
     * @since 1.3
     * @throws UnsupportedEncodingException
     */
    public static byte[] decode(byte[] source) throws UnsupportedEncodingException {
        return decode(source, 0, source.length);
    }

	/**
	 * Decodes web safe Base64 content in byte array format and returns
	 * the decoded data.
	 * Web safe encoding uses '-' instead of '+', '_' instead of '/'
	 *
	 * @param source the string to decode (decoded in default encoding)
	 * @return the decoded data
	 */
	public static byte[] decodeWebSafe(byte[] source)
	throws UnsupportedEncodingException {
		return decodeWebSafe(source, 0, source.length);
	}

	/**
	 * Decodes Base64 content in byte array format and returns
	 * the decoded byte array.
	 *
	 * @param source The Base64 encoded data
	 * @param off    The offset of where to begin decoding
	 * @param len    The length of characters to decode
	 * @return decoded data
	 * @since 1.3
	 * @throws UnsupportedEncodingException
	 */
	public static byte[] decode(byte[] source, int off, int len)
	throws UnsupportedEncodingException {
		return decode(source, off, len, DECODABET);
	}

	/**
	 * Decodes web safe Base64 content in byte array format and returns
	 * the decoded byte array.
	 * Web safe encoding uses '-' instead of '+', '_' instead of '/'
	 *
	 * @param source The Base64 encoded data
	 * @param off    The offset of where to begin decoding
	 * @param len    The length of characters to decode
	 * @return decoded data
	 */
	public static byte[] decodeWebSafe(byte[] source, int off, int len)
	throws UnsupportedEncodingException {
		return decode(source, off, len, WEBSAFE_DECODABET);
	}

	/**
	 * Decodes Base64 content using the supplied decodabet and returns
	 * the decoded byte array.
	 *
	 * @param source    The Base64 encoded data
	 * @param off       The offset of where to begin decoding
	 * @param len       The length of characters to decode
	 * @param decodabet the decodabet for decoding Base64 content
	 * @return decoded data
	 */
	public static byte[] decode(byte[] source, int off, int len, byte[] decodabet)
	throws UnsupportedEncodingException {
		int len34 = len * 3 / 4;
		byte[] outBuff = new byte[2 + len34]; // Upper limit on size of output
		int outBuffPosn = 0;

		byte[] b4 = new byte[4];
		int b4Posn = 0;
		int i = 0;
		byte sbiCrop = 0;
		byte sbiDecode = 0;
		for (i = 0; i < len; i++) {
			sbiCrop = (byte) (source[i + off] & 0x7f); // Only the low seven bits
			sbiDecode = decodabet[sbiCrop];

			if (sbiDecode >= WHITE_SPACE_ENC) { // White space Equals sign or better
				if (sbiDecode >= EQUALS_SIGN_ENC) {
					// An equals sign (for padding) must not occur at position 0 or 1
					// and must be the last byte[s] in the encoded value
					if (sbiCrop == EQUALS_SIGN) {
						int bytesLeft = len - i;
						byte lastByte = (byte) (source[len - 1 + off] & 0x7f);
						if (b4Posn == 0 || b4Posn == 1) {
							throw new UnsupportedEncodingException(
									"invalid padding byte '=' at byte offset " + i);
						} else if ((b4Posn == 3 && bytesLeft > 2)
								|| (b4Posn == 4 && bytesLeft > 1)) {
							throw new UnsupportedEncodingException(
									"padding byte '=' falsely signals end of encoded value "
									+ "at offset " + i);
						} else if (lastByte != EQUALS_SIGN && lastByte != NEW_LINE) {
							throw new UnsupportedEncodingException(
									"encoded value has invalid trailing byte");
						}
						break;
					}

					b4[b4Posn++] = sbiCrop;
					if (b4Posn == 4) {
						outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
						b4Posn = 0;
					}
				}
			} else {
				throw new UnsupportedEncodingException("Bad Base64 input character at " + i
						+ ": " + source[i + off] + "(decimal)");
			}
		}

		// Because web safe encoding allows non padding base64 encodes, we
		// need to pad the rest of the b4 buffer with equal signs when
		// b4Posn != 0.  There can be at most 2 equal signs at the end of
		// four characters, so the b4 buffer must have two or three
		// characters.  This also catches the case where the input is
		// padded with EQUALS_SIGN
		if (b4Posn != 0) {
			if (b4Posn == 1) {
				throw new UnsupportedEncodingException("single trailing character at offset "+ (len - 1));
			}
			b4[b4Posn++] = EQUALS_SIGN;
			outBuffPosn += decode4to3(b4, 0, outBuff, outBuffPosn, decodabet);
		}

		byte[] out = new byte[outBuffPosn];
		System.arraycopy(outBuff, 0, out, 0, outBuffPosn);
		return out;
	}
}
